Varactor folding technique for phase noise reduction in electronic oscillators

Abstract

A varactor folding technique reduces noise in controllable electronic oscillators through the use of a series of varactors having relatively small capacitance. A folding circuit provides control signals to the varactors in a sequential manner to provide a relatively smooth change in the total capacitance of the oscillator. Consequently, effective control of the oscillator is achieved with accompanying reductions in oscillator noise such as flicker noise.

Description

CROSS-REFERENCE TO RELATED APPLICATION[0001]This application is a continuation of U.S. patent application Ser. No. 10,395,116, filed Mar. 25, 2003 now U.S. Pat. No. 6,806,787, which is a divisional of U.S. patent application Ser. No. 09,738,858, filed Dec. 14, 2000 now U.S. Pat. No. 6,563,392, which claims the benefit of the U.S. Provisional Application No. 60,170,840, filed Dec. 14, 1999, all of which are incorporated herein by reference in their entireties.BACKGROUND OF THE INVENTION[0002]Communication systems use oscillators to process various signals used in the communication process. In a typical communication system, an information signal (e.g., a television program) is modulated onto a high frequency carrier signal to facilitate transmission of the signal. Such modulation may be accomplished, for example, by varying the phase of the carrier signal according to the information in the signal. Through the use of different carrier signals of different frequencies, many informatio...

AI Technical Summary

Benefits of technology

[0007]The invention is directed to techniques for reducing noise in electronic oscillators. A folding technique is described for controlling circuit parameters in an electronic oscillator by separately controlling a plurality of circuit elements that affect those parameters. Through the use of a plurality of circuit elements, substantial noise reduction can be achieved. In addition, the use of a folding technique facilitates making smooth transitions when sequentially activating circuit elements. Thus, the folding technique described herein ensures that the combined effect of the adjustable circuit elements results in a relatively smooth adjustment of the circuit parameters.

[0008]In one embodiment the oscillator includes a plurality of varactors in the oscillator's tank circuit. The frequency of the oscillator is controlled, in part, by varying the voltage across the varactors. By using several varactors having relatively small capacitance instead of one varactor having a relatively large capacitance, flicker noise can be substantially reduced in the oscillator. In addition, a folding circuit is utilized to control the varactors so that each varactor is activated in a substantially sequential manner, relative to one another, such that the overall change in capacitance in the tank circuit occurs in a relatively smooth manner, without substantial transient effects.

[0009]In one embodiment, the folding technique takes advantage of a sigmoidal response characteristic of the varactor components. This sigmoidal response characteristic has the effect that the capacitance of the varactor changes in a substantially linear manner when a particular range of voltages are applied across the varactor. Outside that voltage range, however, the capacitance of the varactor remains relatively constant. By properly folding the variable capacitance of each of these varactors into the tank circuit of the oscillator, the invention achieves very low noise characteristics in a controllable oscillator.

[0010]In another embodiment of the invention, a folding circuit sequentially activates varactors in the circuit such that when the circuit is ready to fold a new varactor into the circuit, the folding circuit couples a fixed capacitance to the tank circuit to replace a varactor that is then isolated from the circuit. Again, the folding circuit facilitates smooth transitions in the total capacitance of the oscillator tank circuit.

Problems solved by technology

Fluctuations in the phase of the transmitter or receiver oscillators corrupt the intended phase modulation of the signal in systems which use phase or frequency modulation.

Phase noise may also reduce the ability of a radio system to reject or discern unwanted channels close in frequency to the desired channel.

Nevertheless, system performance is often limited by the quality of available oscillators at practical levels of cost, size, and power dissipation.

Oscillators also may suffer from a significant level of noise due to up-conversion of flicker (1 / f) noise from low frequencies to near the center frequency of the oscillator.

Flicker noise occurs at low frequencies in nearly all electronic components.

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